The present invention relates to novel polyurethane compositions which are advantageous for the production of holographic media, inter alia for data storage, but also for optical applications of different types.
In the structure of holographic media, as described in U.S. Pat. No. 6,743,552, information is stored in a polymer layer which substantially comprises a polymer matrix and very special polymerizable monomers present in uniform distribution therein. This matrix polymer may be polyurethane-based. It is prepared starting from isocyanate-functional materials which are crosslinked with polyols, such as polyethers or polyesters, with urethane formation.
U.S. Pat. Nos. 6,743,552, 6,765,061 and 6,780,546 disclose polyether-based PU matrices for use in holographic media, which are substantially based on trifunctional polypropylene oxides (PPO) and mixtures of poly(THF) with trifunctional PPO. Some of the formulations described there contain PPO having a low molar mass (Mn≦1000 g/mol) as a mixture with poly(THF), also having molar masses up to Mn≦1500 g/mol. A very similar approach is described in JP 2008015154 A 20080124. Here, the matrix was formed from difunctional isocyanates and mixtures of poly(THF) and trifunctional PPO. Likewise, US 2003044690 A1 20030306 describes the synthesis of a PU matrix from Desmodur® N 3400, Desmodur® N 3600 or Baytec WE-180 and a trifunctional PPO based on glycerol, having an Mn of 1000. In addition, in JP 2008070464 A 20080327, polyether-based PU matrices having relatively high Tg values (>30° C.) are used for holographic media. In WO 2008029765 A1 20080313, polyester- and polycarbonate-based polyols are used as components for polyurethane matrices for volume holograms and holographic media. WO 2005116756 A2 20051208 describes low-Tg polyurethane matrices based on a mixture of polyesters and Surfynol 440 (Air Products and Chemicals, Inc., Allentown, USA), a polyether with alkynediol starter for embossed holograms.
Furthermore, the patents JP 2007101743, JP 2007086234, JP 2007101881, US 20070077498 and US 20070072124 describe the use of di- and trifunctional polypropylene oxide in connection with PU matrices in the area of holographic data memories or as “volume-type holographic optical recording media”. The isocyanate component used there was dicyclohexylmethane 4,4′-diisocyanate (“H12-MDI”) or a prepolymer of the abovementioned components, in some cases in the presence of 1,4-butanediol as a chain extender. Analogous formulations are disclosed in the patents JP 2007187968 and JP 2007272044 for the area of “information recording and fixation” and “high density volume holographic recording material”. The patent JP 2008070464 describes an analogous formulation as matrix material for holographic data memories and “holographic recording materials and recording media”. In this case, polyethylene glycol having a number average molar mass (Mn) of 600 g/mol was used as a chain extender and, in addition to “H12-MDI”, hexamethylene diisocyanate was also used. A trifunctional polypropylene oxide in combination with hexamethylene diisocyanate and/or Desmodur® N3300 was described in the patent JP 2007279585 as matrix material for the production of “holographic recording layers” and an “optical recording medium”.
However, a disadvantage of the known polyurethane-based systems, in particular for optical applications outside digital data storage, is that the achievable brightness of the holograms stored in such media is too low. The reason for this is in general that the relative difference between the refractive indices of polyurethane matrix and writing monomer is too small. On the other hand, an arbitrary variation of the matrix polymer is not possible since good compatibility of the matrix polymer with writing monomer and the further components present in the formulations must always be ensured. Furthermore, for processing reasons, it is of interest to ensure that mixing and provision of the formulations are as simple as possible to carry out.
It was therefore an object of the invention to provide novel polyurethane compositions which permit a better contrast ratio and improved brightness of the holograms without sacrifices with respect to the compatibilities of matrix polymer and writing monomer.
It has now surprisingly been found that the abovementioned requirements can be met if special polyether polyols are used for synthesizing the matrix polymer.